Air filters are commonly employed in forced air or blower ventilation systems, including heating and cooling systems. Such systems are used for a variety of purposes, such as home heating and cooling, air circulation and cleaning in closed commercial and office environments, and commercial and industrial heating and cooling applications.
In a typical forced air or blower system, air is taken in through one or more intake vents or ducts, which may be located inside the building for recirculation, or which may intake fresh air from outside the building. Air taken in through these ducts will often contain coarse particulate matter, such as sand, dirt and hair, and also typically includes fine particulate matter such as dust, smoke, germs and similar minute particles. Before the air enters the blower, it is desirable to remove both coarse and fine particles from the air, to prevent the buildup of such unwanted materials in the blower. Removal of particles increases blower life, and reduces maintenance and cleaning costs relating to heating, cooling and ventilation. Particulate removal also enhances air quality, since the undesirable particles are removed from the air stream. In recirculation systems, with little fresh air introduced relative to the total air volume moved through the system, particulate removal is critical to reduce the spread of germs and allergens through the building.
In order to remove particles, residential and commercial forced air and blower systems have previously employed fiberglass filters in intake ducts. These filters are typically comprised of amorphous networks of fiberglass set in or around metal, plastic or paperboard frames. A single density of fiberglass is typically employed to remove both coarse and particulate fibers.
A number of problems, however, are inherent in such fiberglass filters. Single density filters usually have a short lifespan, since a fine network of fibers must be used to trap both fine and coarse particles; coarse particles tend to block the flow of air through the filter as they build up on the filter surface. Another defect found in fiberglass filters relates to recent studies that have determined that fiberglass is a Class B carcinogen. As a result of these deficiencies, other filtering means are needed for use in forced air and blower systems.
Synthetic fibers, such as polyester and polybutylene, have also been employed to create amorphous networks for use in filters for forced air and blower systems. Typically, such filters are made by welding or sealing two sheets of amorphously networked synthetic fibers together around a frame. Welding occurs through the induction of electrical current resulting from a radio frequency (RF) generator associated with a die in a stamping process. In the conventional process, a die is cast in the pattern of the filter to be made. A high amplitude pulse of electromagnetic energy, in the form of a radio wave, is applied to the die, which heats the fibers in the two sheets, causing them to melt and thereby bond to one another. After welding has occurred, the die is removed.
The production of filters using such a process is typically a die-stamping operation, which is relatively slow and results in low production volumes and increased production costs. Die stamping also means that any change in the design of the filter being produced requires the casting and installation of a new die, resulting in lost production due to down time. In addition, variations in the thickness and density of the fiber sheets can results in sections of filter that are melted or burned, rendering the filter unusable. The result of these problems has been as effective limit on the ability to manufacture synthetic air filters in an efficient and cost-effective manner.
The use of ultrasonic welding techniques is well-known in the manufacture of products other than air filters use in HVAC systems. For example, U.S. Pat. No. 4,686,136, describes laminated fabrics and fiber mats produced by welding one or more woven fabrics to a non-woven batt made from carbon fibers and thermoplastic materials. Ultrasonic energy is applied through a sonic horn vibrating against an anvil roller on which numerous pins are disposed. According to the patent, the pins function to concentrate the sonic energy, forming structural columns of material at the points of concentration, and permitting fiber batt to remain uncompressed in areas where energy is not concentrated by an anvil roller pin. Such a technique would not be suitable for air filters, however, since such filters are comprised of at least two non-woven fiber mats, which must be bonded continuously at all margins.
A similar arrangement is shown in U.S. Pat. No. 4,659,614. In that patent, raised surfaces on the anvil roller are used to concentrate ultrasonic energy to produce a quilted fabric made from either a single web of non-woven, thermoplastic fibers, or from a densely packed web of such fibers bonded to a loosely packed web of fibers.
Other examples of thermoplastic welding are illustrated in U.S. Pat. Nos. 4,883,547, 4,713,131, 4,501,782, 3,982,978, 3,817,802, and 3,193,169. The techniques disclosed in those patents are not suitable for the commercial production of air filters, which requires a continuous process to produce a narrow, welded seam regardless of material thickness or density.
None of the methods described in the patents just cited provides an adequate method of producing an air filter of the type described above. The bonding of various thicknesses of an amorphous web of synthetic fibers, to encompass a frame, is neither shown nor suggested by those patents. The techniques described in those patents are therefore unsuitable for use in the manufacture of air filters for HVAC systems.